Kilns



C. D. TINKER May 28, 1963 KILNS 2 Sheets-Sheet 1 Filed Feb. 3. 1958 INVENTOR. CHARLES DEAN 'HNKER ATTORNEY C. D. TINKER May 28, 1963 KILNS 2 Sheets-Sheet 2 Filed Feb. 5, 1958 INVENTOR.

C HAR LES DEAN Tl NKER ATTORNEY United States Patent 0 3,091,444 KILNS Charles Dean Tinker, Granville Township, Licking County, Ohio, assignor to Ohio Kilns, Inc., Granville, Ohio, a corporation of Ohio Filed Feb. 3, 1958, Ser. No. 712,868 7 Claims. (Cl. 263-40) The invention disclosed herein and illustrated in the appended drawings relates generally to furnaces and more particularly to the type of furnaces which are designated as kilns and which are used for calcining or sintering raw materials such as clays, lightweight aggregates, ores and similar mineral substances.

Heretofore sintering of such materials has been accomplished in a variety of ways and other ways not generally used have been proposed. Furnaces of various shapes and kinds have been used. For example, cylindrical kilns constructed of metal, of concrete and of various ceramic materials have been used. Sintcring machines utilizing conveyors or sintering belts have been used. Concrete rotary kilns have been used. Cylindrical kilns having vertical axes have also been proposed.

In some of such prior art furnaces it has been proposed that the material to be sintered be mixed with solid fuel and the mixture fed into the furnace and the fuel there burned in situ.

Some difficulties in such kilns as used and proposed heretofore relate to the conservation of the heat provided for the sintering operation, to the control of the temperatures of the kiln, to the control of the zone of combustion of mixtures of fuel and material to be sintered, and to preventing the material being sintered from sticking to the sides of the kiln.

Where mixtures of fuel and material to be sintered are fed into a furnace and the fuel there burned it is usual to provide a draft of air to support the combustion of the fuel in the furnace. Such kilns employ a unidirectional flow of heated gases to bring the mineral substance being treated up to the proper sintering temperature. After such heated gases are passed over the mineral substance they are commonly exhausted from the furnace with a resultant loss of the heat energy which they contain. Such devices are relatively expensive to operate because of the inefficient utilization of the heat derived from the fuel which is used, and finished products sintered in such furnaces therefore tend to be relatively expensive to produce.

When certain mineral products are heated to normal sintering temperatures, they become ticky and ditficult to handle by known means. In such cases they stick to the sides of the kiln and not only cut down the operating size of the kiln and detract from the efiiciency of the heat supplied to the kiln, but they also waste a large part of the material to be sintered which has adhered to the sides of the kiln.

The sintered mineral products of the rotary kilns presently in use are sometimes relatively strong in structure but are undesirably dense for certain applications, while the sintered mineral substances produced by the sintering belt are structurally weak and undesirably light in density for some purposes. The sintering kiln of my invention produces a semi-strong sintered product with is semi-light in density and better suited to many purposes than the products of other types of sintering furnaces now in use.

The sintering kiln of my invention is further adapted to utilization of solid carbonaceous fuels in such manner that the heat derived therefrom is conserved and used more efficiently than is possible in sintering furnaces or kilns of the types now in general use.

Objects One of the objects of my invention is to provide a sintering kiln whereby the heat generated by the combustion of fuel is more efficiently utilized for sintering materials as for example mineral substances.

Another object of my invention is to provide a kiln for so sintering mineral substances as to produce a sintered product which is structurally semi-strong and yet semi-light in density.

Another object of my invention is to provide in a sintering kiln means for facilitating the removal of sticky sintered products from the furnace.

A further object of my invention is to provide in a sintering kiln means for maintaining so far as possible, a desired temperature and a desired temperature distribution throughout the mass of material being sintered.

A further object of my invention is to provide a sintering kiln in which the fuel is burned in close proximity to the material being sintered and having means for accurately controlling the temperature of the material being sintered and for controlling the relative position within the furnace of the zone of greatest combustion.

Drawings In the drawings which illustrate specific embodiments of my invention:

FIG. 1 is a diagrammatic sectional view in elevation showing the sintering kiln of my invention showing certain of the charging tubes in dotted lines with one of the tubes removed for clarity so that the central charging tube may be indicated;

FIG. 2 is a sectional view taken along the lines 2-2 of FIG. 1 with the charging conduits omitted but showing the bottom of the combustion chamber; and

FIG. 3 is a diagrammatic sectional view in elevation of an alternative embodiment.

Referring to the drawings for a detailed description of my invention, it will be seen that I have shown diagrammatically in FIG. 1, the sintering kiln of my invention which comprises, in combination, a combustion chamber 10, a plenum chamber 20, in gaseous communication with combustion chamber 10, having a plurality of charging conduits such as 23 extending through said plenum chamber 20 and in communication at their forward end with the combustion chamber 10; means for supplying material to said charging conduits; means for at times supplying air under pressure to plenum chamber 20; means for at other times exhausting the gas and other products of combustion from plenum chamber 20', and means for alternately actuating the air supplying and the gas exhausting means.

Combustion chamber 10 in the embodiment shown in FIGS. 1 and 2 is formed of suitable refractory material with the general cylindrical configuration of said FIGS. 1 and 2. It has a grate 11 and an upwardly disposed open discharge opening 12 and a downwardly disposed intake opening 13 in which the grate 11 is arranged transversely of said opening 13 for supporting a mass of fuel and material being processed within chamber 10.

Grate 11 is formed with a plurality of intermediate charging orifices 14, 14a, 14b, 14c, disposed within grate 11, a single center orifice 15, and a plurality of outer orifices 17, 17a, 17b and 17c disposed adjacent the periphery of said grate 11. Grate 11 is also provided with a plurality of draft vents such as 16 arranged between the orifices 14, 14a, 14b, 14c, 15, 17, 17a, 17b and 170.

Plenum chamber 20 is formed with any suitable configuration but is shown as cylindrical. Chamber 20 is further formed with its forward end substantially open and its rearward end 21 substantially closed, said rearward closure having a plurality of openings such as 22 disposed therein for allowing the passage of charging conduits therethrough in substantially draft-tight engagement with end 21. The open forward end of chamber 20 is arranged in draft-tight, butted engagement with opening 13 of combustion chamber so as to be in gaseous communication with draft vents 16 of grate 11 and therefore with the interior cavity of combustion chamber 10.

Extending from the charging (rear) end of the apparatus through the openings 22, through the plenum chamber 20, through the orifices 14, 14a, 14b, 14c, 15, 17, 17a, 17b and 17c are a plurality of charging conduits shown partly in broken lines in FIG. 1. These conduits are preferably formed with a generally cylindrical configuration. Central charging conduit 23 extends through the orifice and four intermediate conduits extend through the orifices 14, 14a, 14b, and 140. Two of the intermediate charging conduits are designated 24 and 25 in FIG. 1, and extend respectively through orifices 14c and 14b. The two opposite intermediate charging conduits are hidden behind conduits 24 and 25. There are four outer charging conduits which extend through the orifices 17, 17a, 17b and 17c. Two of the outer charging conduits are designated 26 and 27 respectively. The nearer outer charging conduit is omitted in FIG. 1 so that the conduit 23 may be shown in dotted lines. The fourth outer charging conduit is hidden behind conduit 23. A laterally disposed opening 28 is also provided in plenum chamber 20 for receiving an air supply from exhaust duct 56 described hereafter in detail.

I provide means for supplying various mixtures of unsintered material and solid fuel to the chamber 10 through the central and intermediate charging conduits. This means which I have designated generally as 30 comprises a plurality of charging plungers or rams operatively positioned in the central and intermediate charging conduits. For instance, I provide the plunger 31 having a stern 31a and a piston head 31b locate-d within the conduit 23. I provide a hydraulic motor 31c for actuating the plunger 31. Similarly I provide plungers 32 and 33 respectively in charging conduits 24 and 25 each being provided with a stem 32a and 33a and with a piston head 32b or 33!; respectively and actuated by hydraulic motors 32c and 330 respectively. The means for supplying such mixtures also includes a plurality of material hoppers such as for example, hoppers 34 and 35 shown in FIG. 1. Hoppers 34 and 35 are formed with a configuration suitable for containing a supply of granulated material to be sintered and/or a supply of solid fuel in granulated form. Each of hoppers 34 and 35 are arranged in communication with the central bore of at least one of the charging conduits, so that the material contained within hoppers 34 and 35 is continuously fed by the force of gravity into the bore and said charging conduits. Thus the hopper 34 is connected through port 37 with conduit 23 and hopper 35 is connected with conduits 24 and 25 through ports 38 and 39 respectively.

The charging plungers or rams 31, 32 and 33 are slidably retained within charging conduits such as conduits 23, 24 and 25 and each is reciprocally driven therein by its associated reciprocating differential motors 31c, 32c and 33c respectively.

I provide means for supplying a previously sintered product through the outer charging conduits such as 26 and 27 to the chamber 10. I have designated this means as 40 and it includes as shown one or a plurality of hoppers 41 formed as shown in FIG. 1 with a configuration suitable for containing a supply of a sintered product. Each hopper 41 feeds continuously by gravity into the bore of one of the outer charging conduits. In each of the outer charging conduits there is positioned a charging plunger or ram such as the plungers 42 and 43 positioned respectively in the conduits 26 and 27. Each plunger is provided with a stem, :1 piston head and a suitable reciprocating differential motor. Such motor may be hydraulic as shown or electric, or any other suitable type. Thus the plunger 42 is provided with a stem 42a, a piston head 42b and a motor 420. The plunger 43 is provided with a stem 43a, piston head 43b and a motor 43c.

I provide means for at times supplying air under pressure to plenum chamber 20. This means which I have designated generally as 50 comprises a centrifugal blower 53, driven by a suitable motor 52, and an appropriate duct connecting the output of blower 53 to a manifold 56 leading to the lateral draft opening 28 in plenum chamber 20 which is previously described herein.

I provide means for at other times exhausting gases from plenum chamber 20. This means which I have designated generally as 60 comprises a centrifugal exhauster 63, driven by a suitable motor 62, and an appropriate exhaust duct connecting the inlet orifice of blower 63 to the manifold 56 and thus to the draft opening 23 in plenum chamber 20.

I provide means for controlling the effectiveness of means 50 and means 60 so as to make means 50 at times effective and alternately to make means 60 at other times effective. This means I have designated generally as 70. In the preferred embodiment of my invention, illustrated in FIG. 1, I show this means to include air supply damper 57, of the conventional butterfly type, pivotally secured within supply duct 55, and exhaust damper 67 of similar configuration pivotally secured within exhaust duct 65. Dampers 57 and 67 are each provided with actuating arms 58 and 68, respectively, each of said arms being in fixed engagement with its respective damper. The outer ends of arms 57 and 67 are interconnected by a suitable link 71 whereby longitudinal movement of link 71 serves simultaneously to open one of dampers 57 and 67 and to close the other. Although link 71 can be repositioned longitudinally for simultaneously actuating means 50 and means 60, either by manual or by other well known motor means, I prefer to provide an appropriate damper motor 72 for repositioning link 71 and thereby actuating dampers 57 and 67. The means 70 may alternatively (if desired) consist of a double throw electric switch (not shown) of conventional design which functions simultaneously to deenergize one of the motors 52 and 62 and to energize the other.

Damper motor 72 may be energized manually, as desired, to vary the direction of flow of air and exhaust gases to and from combustion chamber 10 through plenurn chamber 20. Alternatively, automatic cycling means (not shown) may be employed to reverse the flow through plenum chamber 20 as frequently as desired.

Suitable means for controllably varying the volume of air which is permitted to pass through blower duct 55 and exhaust duct 65 may be provided, if desired, in order to controllably vary the volume of air per unit time which is passed through chamber 10, and thus to compensate for the variations which are normally encountered in the volume of air required for complete combustion of a given carbonaceous fuel. In my preferred embodiment shown in the drawings I provide for such purposes inlet damper 85, of the well known slide gate type, located in the inlet duct 65 of exhauster 63 and a similar damper (not shown) located in the inlet duct (not shown) of blower 53. Such inlet dampers may be controlled, as desired, by well known means, either individually or in concert to regulate the volume of air which is passed through chamber 10 in a given time.

In FIG. 3 I have shown an alternate embodiment of my invention. Therein is shown a combustion chamber having a grate 111 and an open discharge opening 112. Positioned below the grate 111 is a plenum having a central charging conduit 123, and a plurality of intermediate charging conduits (of which only two designated 124 and 125 are shown) passing through said plenum. Around the periphery of the chamber 110 there are connected a plurality of outer charging conduits (of which only two designated as 126 and 127 are shown) not passing through the plenum but provided for the purpose of supplying previously sintered material serving as a heat insulation for the walls of the combustion chamber and as a carrier which carries the sticky material being sintered and thus keeping such material from adhering to the walls. At such times part of the previously sintered material slides on another portion of the previously sintered material, the sticky material being carried by the sliding previously sintered material.

Central conduit 123 is fed from raw material and fuel hopper 134. Intermediate conduits 124 and 125 are fed by raw material and fuel hopper 135 and outer conduits 126 and 127 are preferably fed by sintered material hopper 141 although with some types of material which does not tend to become sticky raw material and/ or fuel could also be fed from hopper 141. Conduits 123, 124, 125, 126 and 127 are provided with screw conveyors 131, 132, 133, 142 and 143 respectively and these conveyors are driven by electrical motors such as the motor 131C. Air is forced into the plenum 120 and thence into the combustion chamber 110 by the valve gear and fan 170 through the air conduit 156 and is withdrawn from the chamber 110 through the plenum 120 also through the air conduit 156 and also by the valve gear and fan 170.

Not only is the temperature of the material in the chamber 110 controlled by the alternate cycling of blowing and exhaust controlled by the valve gear and fan 170, but also the position of the zone of highest heat in the furnace is controlled thereby. Thus when the fan is on exhaust the temperature falls and the zone of greatest heat is lowered toward the grate 111. When the fan is blowing to force air into the plenum and to the furnace chamber the temperature rises and the zone of greatest heat moves upward from the grate 111. By frequent and controlled reversal of blowing and exhaust I keep the combustion zone from rising too high in the furnace. The result of this is that the temperature and the zone of greatest heat can be kept substantially uniform. When the fan is on exhaust, the plenum serves to preheat the raw material and during such times serves as a heat exchanger to conserve the heat obtained from the fuel. Thereafter on reversal of the flow the combustion air is preheated. I prefer to use a time cycle reversing the valve gear and fan often and periodically modifying the times of the cycle to obtain most efficient utilization of the fuel. Normally the time of the blowing phase will not equal the time of the exhaust phase in the cycle. I prefer to superimpose on the time cycle a control based on the temperature in the furnace as sensed by a temperature sensing unit.

Operation In the operation of the furnaces shown in the illustrations of my invention for sintering mineral substances, the material to be sintered, in granulated form, is preferably mixed with a granulated solid fuel, such as, for example, powdered coal or other carbonaceous fuel, in proportions appropriate for generating combustion temperatures within chamber which are slightly higher than the optimum temperature for the proper sintering of the mineral substance. A suitable supply of such high fuel content mixture, hereafter referred to as the rich mixture, is deposited in one or more hoppers, such as for example, hopper 34.

A second mixture of granulated material and granulated fuel is also prepared having a fuel content less than that required to generate an optimum sintering temperature in chamber 10. A suitable supply of such low fuel content mixture, hereafter referred to as the lean mixture, is deposited in other hoppers, such as for example, hopper 35. The mixtures contained in hoppers 34 and 35 are fed by gravity into the respective conduits such as conduits 23 and 24 and 25 which are connected as previously described to hoppers 34 and 35.

A supply of previously sintered material (in granulated form) is deposited in hoppers such as hopper 40. The sintered materials contained in various hoppers 40 are fed by gravity into respective outer conduits such as the conduits 26 and 27.

The appropriate mixture conveying means and all of the sintered product conveying means are then simultaneously actuated for conveying a rich fuel mixture through the charging orifices such as for example the orifices 15, 14, etc. into the central area of the chamber 10, and for conveying the sintered product through the outer orifices 17, etc. and into the peripheral area of chamber 10. The previously sintered material thus introduced into chamber 10 through orifices 17, etc. characteristically has heat conductivity properties somewhat lower than those of the unsintered material contained in the central area of chamber 10. Therefore the simultaneous introduction of such material through the plurality of outer orifices 17, etc. serves to form a boundary layer of refractory material which constitutes an insulating heat barrier adjacent the perimetrical Wall of chamber 10 for reducing heat losses which would otherwise occur through the wall. Both the insulating boundary layer of pre-sintered material and the central core of material in process are urged upwardly within chamber 10 by the force of new supplies of the various materials which are conveyed into the lower portion of fur nace through outer orifices 17, etc. and charging orifices 14 and 15, etc., respectively. When a quantity of the rich mixture has thus been introduced into the central portion of chamber 10, the fuel component thereof is ignited by any appropriate method and the continuous process of sintering the sinterable mineral component of the mixtures is thus begun.

When the continued combustion of the fuel component of mixtures subsequently introduced into chamber 10 generates temperatures within chamber 10 which are in excess of the optimum temperature for carrying out the sintering process, then those conveyors 31 which serve to feed fuel-rich mixtures from hoppers (such as hopper 34) are deactuated; and other conveyors 32 and 33 appropriate for conveying fuel-lean mixtures from hoppers (such as hopper 35) are actuated. Fuel-lean mixtures are thereafter conveyed to chamber 10 until the temperature within chamber 10 falls slightly below the optimum sintering temperature, whereupon the process just described is reversed so as to supply again a relatively fuel-rich mixture to chamber 10. Thus by actuating appropriate motors 31c, 320, and/or 33c, for conveying relatively rich or relatively lean mixtures to chamber 10 in response to decreases and increases, respectively, in, the temperatures occurring within chamber 10, a relatively constant optimum sintering temperature is main tained therein.

Further control of the rate of combustion and the ultimate temperatures attained within chamber 10 is afforded by the reversing and/or cycling of the direction of air flow through chamber 10. When link 71 is positioned in its uppermost position (FIG. 1) damper 57 is moved to its fully closed position and serves to obstruct the passage of air from blower 53 through duct 55 to plenum chamber 20. With link 71 positioned in its uppermost position (FIG. 1) damper 67 is in the fully open position and air is permitted to fiow from plenum chamber 20 through ducts 56 and 65 and is exhausted by exhauster 63. When gases are thus exhausted from plenum chamber 20 by exhauster 63, the gaseous products of the combustion within chamber 10 are drawn from chamber 10 through vents 16 and into chamber 20 and relatively cold outside air is simultaneously drawn into chamber 10 through upper opening 12. Such flow of air and gases downwardly through chamber 10 tends to decrease the temperature of those materials contained therein which are located adjacent opening 12, and to increase the temperature of the material contained below the hottest zone in chamber 10 and above grate 11. Thus, the downward movement of air and gases tends to move the region of the highest temperature occurring within chamber 10 downwardly. Excessive How of air downward may move this zone to an undesirable position immediately above grate 11. I

When link 71 is repositioned to its lowermost position damper 67 is rotated through arm 68 to the fully closed position, blocking any movement of gases from chamber through exhauster 63; and damper 57 is rotated through arm 58 to its fully open position. With dampers 57 and 67 in the relative positions just described, air from blower 53 is permitted to flow through ducts 55 and 56, into plenum chamber 20, and upwardly through vents 16 into combustion chamber 10; thus exhausting the gaseous products of combustion through discharge opening 12 and moving the zone of highest temperature within chamber 10 (in the manner previously described) upwardly away from grate 11 and toward opening 12.

By reversing the flow of air and gases through chamber 10 in the manner just described, at frequent intervals, the zone of highest temperature occurring within chamber 10 is confined to an area having predeterminable limits intermediate the upper opening 12 and lower opening 13 of chamber 10. A zone of relatively constant temperature is thus maintained in the central area of chamber 10.

The reverse cycling just described also serves to draw hot gases from chamber 10 into chamber 20, at measured intervals, which serve to heat those portions of charging conduits such as 23, 24 and 25 which are contained within plenum chamber 20. When materials are passed through the heated charging conduits such as 23, 24 and 25 en route to chamber 10 they are preheated by contact with the heated conduits and are thus subsequently introduced into chamber 10 in a preheated condition. Similarly, when the flow of air through chamber 20 is reversed and outside air is introduced into chamber 20 by blower 53, the air so introduced is preheated by contact with the outer wall of chamber 20 and with charging conduits and is subsequently introduced into; chamber 10 through vents 16 in the preheated state. The purpose of preheating the air and materials introduced into chamber 10 through intake opening 14 is to minimize the heat losses which would otherwise occur within the lower portion of chamber 10 when relatively cold materials and air are introduced into chamber 10, and to thereby effectively increase the area within chamber 10 wherein sintering temperatures are constantly maintained and to economize on the amount of heat necessary for the sintering operations thus saving fuel.

One additional reason for the provision of means for controlling the temperature of the furnace and the position of the hottest zone thereon by varying the mixture of the raw material and fuel supply and by cycling the alternate exhaust and forced draft is the fact that there are many other factors affecting the temperatures in the sintering or calcining of raw materials. For example, the raw material and the fuel will themselves vary as to the heat generated in the sintering or calcining process.

It will be apparent to one schooled in the art that the furnace of my invention can be constructed in various sizes and shapes and any desired number or combination of hoppers and charging ducts may be provided Without appreciably lowering its operating efficiency, since the principle of heat transfer which is relied upon is that of transfer by conduction rather than radiation.

In the main, I have described the embodiment of my invention as intended for the use of solid fuel. Other fuels such as oil or gas may be used. Also, the controls for the blower and/or exhaust fans have been described as electrical. Instead, I might use air or hydraulic motors and relays.

The furnace of my invention can function to provide extremely high temperatures where such temperatures are desired for processing materials having highly refractory characteristics, since a quantity of the refractory material in a finished state is utilized as an insulating outer shell surrounding the central core of material in process. For illustration purposes, the operation of my furnace has been described herein in terms of the most suitable method for sintering or calcining lightweight aggregate materials. However, there are certain ores and other refractory materials which can be processed with equal effectiveness and economy in the furnace of my invcntion.

It is to be understood that the above described embodiments of my invention are for the purposes of illustration only and various changes may be made therein without departing from the spirit and scope of my invention.

I claim:

I. A sintering furnace comprising a combustion cham her; a hopper for supplying a combination of fuel and material to be sintered; a plenum chamber positioned beneath said combustion chamber and in gaseous communication therewith; a conduit connected to said hopper, passing through said plenum chamber, and leading from the lower part of said hopper for supplying mixed fuel and raw material from said hopper to said combustion chamber and means for drawing air through said plenum chamber from said combustion chamber around the portion of said conduit positioned in said plenum chamber.

2. A sintering furnace comprising a combustion chamher, a plenum chamber beneath said combustion chamber and in gaseous communication therewith; means comprising a conduit passing upward through said plenum chamber for supplying a mixture of fuel and raw material to be sintered to the lower part of said combustion chamber; means for forcing combustion air through said plenum chamber and upward therefrom through said combustion chamber; and means for drawing heated products of combustion downward through said plenum chamber from said combustion chamber; and means for reversing the flow to at times force combustion air through the plenum chamber and upward and at times draw heated products of combustion downward through said plenum chamber; whereby the contents of said conduit are and at times heated by the downward passage of heated products of combustion and at times cooled by the upward passage of combustion air.

3. A kiln for sintering a mineral substance comprising a combustion chamber; a plurality of charging conduits connected at one end to said combustion chamber; a plenum chamber connected to said combustion chamber and disposed about a portion of the length of said conduits; a hopper connected to the other end of each of said conduits; means disposed within each said conduit for urging a mixture, composed of said substance and a solid fuel, through each of said conduits; means for supplying air under pressure to said combustion chamber; means for exhausting gases from said combustion chamber; means for reversing the flow of said air and gas to at times actuate said supplying means and at times to actuate said exhausting means; means for varying the ratio of fuel to said mineral substance within said furnace; and means for introducing previously sintered material into said combustion chamber in a zone intermediate said substance and the walls of said combustion chamber.

4. A kiln for sintering a mineral substance comprising a combustion chamber provided with a discharge opening and an intake opening; a grate disposed across said intake opening and formed with a plurality of peripheral charging orifices adjacent the outer edge of said grate, a plurality of central charging orifices adjacent the center of said grate and a plurality of draft vents arranged in said grate intermediate said charging orifices; a plenum chamber formed with an open end and a substantially closed end disposed oppositely from said open end. said open end being in draft-tight communication with said intake opening and said closed end being provided with a plurality of conduit orifices axially aligned with said charging orifices, said plenum chamber being further provided with a plurality of laterally disposed draft openings; a plurality of conduits, each extending through said plenum chamber and being secured within one of said conduit orifices and within one of said charging orifices; a plurality of hoppers each being in inter-communication with one of said conduits; means for conveying said mineral substance through each of said conduits; means for introducing into said plenum chamber a combustion supporting fluid under pressure, said means comprising a blower and a fluid supply conduit connected at one of its ends to said blower and at its opposite end to one of said draft openings; means for exhausting fluid from said plenum chamber comprising an exhaust fan and an exhaust conduit connected at one of its ends to said exhaust fan and at its opposite end to one of said draft openings; air supply controlling means, comprising a supply damper, associated with said blower for controlling the flow of fiuid through said blower; separate exhaust controlling means, comprising an exhaust damper, associated with said exhauster for controlling the flow of fluid through said exhauster; and actuating means for opening one of said controlling means and simultaneously closing the other.

5. A kiln for sintering a granulated mineral substance comprising a combustion chamber having a discharge opening and an inlet opening and including a grate disposed iacross said inlet opening and provided with a plurality of peripheral charging orifices adjacent its outer edge, a plurality of central charging orifices arranged adjacent the center of said grate, and a plurality of draft vents disposed in said grate intermediate said charging orifices; means comprising a plurality of mixture hoppers for separately containing a plurality of mixtures of said mineral substance and a granulated solid fuel, said mixtures being composed of different proportions of said mineral substance and said solid fuel; means comprising a plurality of mixture conduits each of said mixture conduits being in communication with at least one of said mixture hoppers and with one of said central orifices for at times conducting one of said mixtures from said hop-per to said combustion chamber; means comprising a plurality of product hoppers for containing separate quantities of said mineral substance in the previously sintcred state; means comprising a plurality of product conduits each of said product conduits being in communication with at least one of said product hoppers and with one of said peripheral orifices for conducting said sintered substance from said product hoppers to a peripheral zone within said combustion chamber; means comprising a plurality of conveyors one of said conveyors being disposed within each of said conduits for forcibly urging said sintered material and said mixtures from said hoppers and into said combustion chamber; means for containing said conduits in an atmosphere of controlled temperature, said means comprising a plenum chamber disposed in draft tight engagement about said conduits and provided with at least one lateral opening and an axial opening connected to said intake opening; means comprising a blower connected to said lateral opening for at times supplying air under pressure to said plenum chamber; means comprising an exhauster connected to said lateral opening for at times exhausting gases from said plenum chamber when air is not supplied there-to; and means comprising a plurality of dampers arranged intermediate said lateral opening and said blower and said exhauster for controlling the alternate how of air to and from said lateral opening.

6. An apparatus for sintcring raw material comprising a cylindrical furnace through which material to be sintered passes while being sintered and which is provided at its lower inlet end with a grate; means, comprising a plurality of tubes and a plurality of screw conveyors within said tubes, for supplying a mixture of raw material and fuel to said furnace adjacent to the central portion of said cylindrical furnace, said tubes at their outlet end extending into said furnace past the grate thereof; and means for supplying previously sintered material to said furnace adjaccnt to the periphery of said cylindrical furnace comprising additional tubes entering into and extending into said furnace adjacent the periphery thereof and extending into said furnace past the grate thereof whereby the mixture of raw material and fuel is burned in the center of said furnace and the heat of combustion is insulated from the periphery of said cylindrical furnace by previously sintered material, and whereby the relatively sticky raw material being sintered is separated from the periphery of the furnace by the previously sintered material and slides through said previously sintered material in its passage through said furnace.

7. An apparatus for sintering raw material comprising a cylindrical furnace through which material to be sintered passes while being sintered and which is provided at its lower inlet end with a grate; means, comprising a plurality of tubes extending up into said furnace past the grate thereof, for supplying a mixture of raw material and fuel to said furnace adjacent to the central portion of said cylindrical furnace; and means for supplying previously sintered material to said furnace adjacent to the periphery of said cylindrical furnace comprising additional tubes entering into and extending upward into said furnace past the grate thereof adjacent the periphery thereof whereby the mixture of raw material and fuel is burned in the center of said furnace and the heat of combustion is insulated from the periphery of said cylindrical furnace by previously sintered material, and whereby the relatively sticky raw material being sintered is separated from the periphery of the furnace by the previously sintered material and slides through said previously sintercd material in its passage through said furnace.

References Cited in the file of this patent UNITED STATES PATENTS 1,190,693 Wendt July 11, 1916 1,602,834- Queneau Oct. 12, 1926 2,148,052 Ahlmann Feb. 21, 1939 2,380,056 Lloyd July 10, 1945 2,421,902 Neuschotz June 10, 1947 2,654,594 Somogyi Oct. 6, 1953 2,668,042 Meyer et al. Feb. 2, 1954 2,821,469 Davis Jan. 28, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 O9l 444 May 28 1963 Charles Dean Tinker It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 64, for "with" read which column 3, line 54, for "and" read of Signed and sealed this 10th day of December 1963.

(SEAL) Attest:

EDWIN L, REYNOLDS ERNEST We SWIDER Attesting Officer Ac ting Commissioner of Patents 

1. A SINTERING FURNACE COMPRISING A COMBUSTION CHAMBER; A HOPPER FOR SUPPLYING A COMBINATION OF FUEL AND MATERIAL TO BE SINTERED; A PLENUM CHAMBER POSITIONED BENEATH SAID COMBUSTION CHAMBER AND IN GASEOUS COMMUNICATION THEREWITH; A CONDUIT CONNECTED TO SAID HOPPER, PASSING THROUGH SAID PLENUM CHAMBER, AND LEADING FROM THE LOWER PART OF SAID HOPPER FOR SUPPLYING MIXED FUEL AND RAW MATERIAL FROM SAID HOPPER TO SAID COMBUSTION CHAMBER AND MEANS FOR DRAWING AIR THROUGH SAID PLENUM CHAMBER FROM SAID COMBUSTION CHAMBER AROUND THE PORTION OF SAID CONDUIT POSITIONED IN SAID PLENUM CHAMBER. 